How to let grow PingPong balls inside an invisible box?
You can achieve a similar effect with builtin physics engine.

Just add a bunch of spheres to your scene, set physics type to Rigid Body, Active and collision shape to Sphere. Set it up in advance before duplicating, or copy it afterwards.
Place them inside an invisible container with a simple box-like shape, like the default cube.
Now you need to push them against a wall. Do so by either animating the container shape, or optionally using a second animated object to push them around.

The illustrated example uses an animated squashing box. In retrospective you could probably animate its scale instead, avoiding changing shapes and expensive mesh morphing physics, but this example uses shape keys.
Set the cube mesh up with Rigid Body type physics, with both Dynamic and Animated active, and collisions set to Mesh type, Source as Deform and Deforming option active.

Add one additional shape key besides Base, for that shape key move the back face forward enough distance to squash the desired amount of spheres against the front face. Keyframe the factor for that shapekey from $0$ to $1$ for the desired duration of the animation.